The identification of lone-parent households is a challenging task in an era of new family forms and related changes in the legislation regulating parental care and financial responsibilities. Official statistics hardly reflect the composite reality of households between which children circulate and in which relationships with biological and non-biological parents change during the life course. On the basis of an explorative qualitative study in Switzerland, we focus on the blurry boundaries used to define the transition to lone parenthood. We analyse the life course of individuals whose entry into the lone-parent state fits one of these patterns: (a) becoming a lone parent despite beginning parenthood within a couple (separation, divorce, or widowhood); (b) becoming a lone parent coinciding with becoming a parent (contraceptive failure, partner refusal of parenthood, or lone parenthood as a life choice). We explore the way in which individuals describe their transition to lone parenthood and to which extent they identify themselves as lone parents. We conclude that such transitions are often non-linear processes and are often characterized by much ambivalence. They involve an array of heterogeneous and multidirectional transitions that are not always captured by current surveys, and we suggest how we could improve future measurements of the timing and occurrence of lone parenthood.